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SA604A Datasheet(PDF) 8 Page - NXP Semiconductors |
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SA604A Datasheet(HTML) 8 Page - NXP Semiconductors |
8 / 10 page Philips Semiconductors Product specification NE/SA604A High performance low power FM IF system 1994 Dec 15 404 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 NE 604A 430 430 SR00320 Figure 10. Crystal Input Filter with Ceramic Interstage Filter forms a divider from the output of the limiters back to the inputs (including RF input). If this feedback divider does not cause attenuation greater than the gain of the forward path, then oscillation or low level regeneration is likely. If regeneration occurs, two symptoms may be present: (1)The RSSI output will be high with no signal input (should nominally be 250mV or lower), and (2) the demodulated output will demonstrate a threshold. Above a certain input level, the limited signal will begin to dominate the regeneration, and the demodulator will begin to operate in a “normal” manner. There are three primary ways to deal with regeneration: (1) Minimize the feedback by gain stage isolation, (2) lower the stage input impedances, thus increasing the feedback attenuation factor, and (3) reduce the gain. Gain reduction can effectively be accomplished by adding attenuation between stages. This can also lower the input impedance if well planned. Examples of impedance/gain adjustment are shown in Figure 9. Reduced gain will result in reduced limiting sensitivity. A feature of the NE604A IF amplifiers, which is not specified, is low phase shift. The NE604A is fabricated with a 10GHz process with very small collector capacitance. It is advantageous in some applications that the phase shift changes only a few degrees over a wide range of signal input amplitudes. Stability Considerations The high gain and bandwidth of the NE604A in combination with its very low currents permit circuit implementation with superior performance. However, stability must be maintained and, to do that, every possible feedback mechanism must be addressed. These mechanisms are: 1) Supply lines and ground, 2) stray layout inductances and capacitances, 3) radiated fields, and 4) phase shift. As the system IF increases, so must the attention to fields and strays. However, ground and supply loops cannot be overlooked, especially at lower frequencies. Even at 455kHz, using the test layout in Figure 3, instability will occur if the supply line is not decoupled with two high quality RF capacitors, a 0.1 µF monolithic right at the VCC pin, and a 6.8µF tantalum on the supply line. An electrolytic is not an adequate substitute. At 10.7MHz, a 1 µF tantalum has proven acceptable with this layout. Every layout must be evaluated on its own merit, but don’t underestimate the importance of good supply bypass. At 455kHz, if the layout of Figure 3 or one substantially similar is used, it is possible to directly connect ceramic filters to the input and between limiter stages with no special consideration. At frequencies above 2MHz, some input impedance reduction is usually necessary. Figure 9 demonstrates a practical means. As illustrated in Figure 10, 430 Ω external resistors are applied in parallel to the internal 1.6k Ω load resistors, thus presenting approximately 330 Ω to the filters. The input filter is a crystal type for narrowband selectivity. The filter is terminated with a tank which transforms to 330 Ω. The interstage filter is a ceramic type which doesn’t contribute to system selectivity, but does suppress wideband noise and stray signal pickup. In wideband 10.7MHz IFs the input filter can also be ceramic, directly connected to Pin 16. In some products it may be impractical to utilize shielding, but this mechanism may be appropriate to 10.7MHz and 21.4MHz IF. One of the benefits of low current is lower radiated field strength, but lower does not mean non-existent. A spectrum analyzer with an active probe will clearly show IF energy with the probe held in the proximity of the second limiter output or quadrature coil. No specific recommendations are provided, but mechanical shielding should be considered if layout, bypass, and input impedance reduction do not solve a stubborn instability. The final stability consideration is phase shift. The phase shift of the limiters is very low, but there is phase shift contribution from the quadrature tank and the filters. Most filters demonstrate a large phase shift across their passband (especially at the edges). If the quadrature detector is tuned to the edge of the filter passband, the combined filter and quadrature phase shift can aggravate stability. This is not usually a problem, but should be kept in mind. Quadrature Detector Figure 7 shows an equivalent circuit of the NE604A quadrature detector. It is a multiplier cell similar to a mixer stage. Instead of mixing two different frequencies, it mixes two signals of common frequency but different phase. Internal to the device, a constant amplitude (limited) signal is differentially applied to the lower port of the multiplier. The same signal is applied single-ended to an external capacitor at Pin 9. There is a 90 ° phase shift across the plates of this capacitor, with the phase shifted signal applied to the upper port of the multiplier at Pin 8. A quadrature tank (parallel L/C network) permits frequency selective phase shifting at the IF frequency. This quadrature tank must be returned to ground through a DC blocking capacitor. The loaded Q of the quadrature tank impacts three fundamental aspects of the detector: Distortion, maximum modulated peak deviation, and audio output amplitude. Typical quadrature curves are illustrated in Figure 12. The phase angle translates to a shift in the multiplier output voltage. |
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